1.6–2.1 GHz broadband Doherty power amplifiers for LTE handset applications

Abstract

A Doherty power amplifier (PA) for long term evolution (LTE) applications is fully integrated on a 1.4×1.4 mm2 die using a 2-µm InGaP/GaAs hetero-junction bipolar transistor (HBT) process. The quarter-wavelength transformer is the bandwidth (BW) limit of the Doherty PA. Other bandwidth limiting factors are analyzed and eliminated. A conventional phase compensation circuit and an additional offset line are merged into the input network of the Doherty PA and the output circuit of the peaking amplifier, respectively. The multi-section output matching having the same impedance transformation and a broadband input matching are utilized. A Wilkinson power divider is transformed into a lumped type and integrated on a chip for enhancing the bandwidth of Doherty operation. For a LTE signal with a 7.5-dB peak-to-average power ratio (PAPR) and a 10-MHz BW, the PA with a supply voltage of 4.5 V delivers a power-added efficiency (PAE) of 36.3%, and an adjacent channel leakage ratio (ACLR) of −32 dBc with an average output power of 27.5 dBm at a frequency of 1.85 GHz. Across 1.6–2.1 GHz, the PA performs a PAE of over 30%, a gain of over 28 dB and an ACLR of below −31 dBc at an average output power of 27.5 dBm while satisfying the standard spectrum mask. These results verify that the proposed bandwidth enhancement techniques are effective for the handset Doherty PA.